A true ounce is 28.5 grams. A Mexican Ounce is 25 grams (not 24 grams as incorrectly defined above). The reason the measurement is shorted is because it is easier to divide and multiply by 25 rather than 28.5 when converting between imperial and metric measurements. That way when it is split from larger quantities like kilos or pounds, it divides into even smaller quantities and there is no remainder.